Polypropylene fibers and yarns are well-known and are widely used in textile and other applications owing to a desirable combination of features. These include low cost, ease of processing, strength, chemical inertness and hydrophobicity. Examples of textile applications for the fibers and yarns include backing fabrics and pile or face yarns for carpets; upholstery fabrics; geotextile fabrics; walkover fabrics; automotive fabrics, such as carpets, trunk liners and kick panels; diaper cover stock; and apparel fabrics.
There is, and has long been, a need for improved polypropylene fibers and yarns for use in applications in which the fibers, yarns or textile products containing them are subjected to bending, creasing, wrinkling, compression and the like. Examples include apparel fabrics, fiberfill, carpets, upholstery fabrics and automotive fabrics. Poor resilience of fibers and yarns used in such applications can result in limited recovery from forces to which fibers and yarns are subjected in use and, in turn, poor aesthetics and wear. These may limit or even preclude utility for certain end uses.
A practical example of the impact of resilience on utility of fibers and yarns for a given purpose is provided by experience in the United States carpet industry. Deficient resilience of carpet face yarn leads to poor thickness retention and recovery of pile height after application of compressive forces, such as those resulting from foot traffic and placement of furniture. Other things being equal, carpet with less resilient face yarn will take on a matted and clumped appearance, show wear and need to be replaced more rapidly and to a greater extent than that tufted with more resilient yarns. Similarly, depressions caused by placement of furniture or other objects will recover more slowly, if at all, in carpets with face yarn of low resilience.
These problems have long been recognized and many attempts at solving them have been advanced over the years. Modified polymer compositions and crystallinities have been proposed by polymer producers. Enhanced fiber spinning processes and yarn treatments have been explored by yarn manufacturers. Carpet manufacturers have developed modified carpet constructions. Despite these efforts and advances resulting from some of them, the long felt need for polypropylene fibers and yarns of improved resilience continues. Indeed, despite a combination of cost, colorfastness, stain resistance, mold and mildew resistance and ease of cleaning that is superior to other carpet face yarns, commercial success of polypropylene yarns in the carpet industry has been elusive. The yarns account for only about 25% of overall carpet face yarn usage and considerably less in residential carpets.
In greater detail, carpet sales in the United States in 1992 were about 1.25 billion square yards according to the Carpet & Rug Institute. Of that total, about 65% was so-called residential carpet, e.g., for housing. The balance was so-called commercial carpet, e.g., for office buildings, schools, stores and airports. Major face yarn types currently used for both types of carpet are nylon yarns, normally composed of polyepsilon-caprolactam or polyhexamethylene adipamide, also known as nylon 6 and 66, respectively; polyester yarns, normally composed of polyethylene terephthalate; and polypropylene yarns, typically composed of crystalline homopolymer polypropylene. In 1993, according to Carpet & Rug Industry Review, October, 1993, total United States carpet face yarn sales were about 2.9 billion pounds, with nylon yarns accounting for about 65%, polypropylene yarns accounting for about 25%, and polyester yarns accounting for about nine %. Wool, cotton and other yarns accounted for less than one %.
Nylon yarns are and have been the dominant synthetic face yarns for both residential and commercial carpets. However, polypropylene yarns' usage in commercial carpets has increased from essentially nothing in the late 1970's to about 40% as of 1992. This growth can be attributed to a combination of factors. One is the superior performance characteristics noted above. In addition, polypropylene yarns typically are less costly than nylon yarns, not only in cost per unit weight of yarn, but even more so in cost per unit area of coverage, because, for a given yarn type, polypropylene yarns have lower density than nylon yarns. The volume of a given weight of polypropylene yarn exceeds that of the same weight of nylon yarn; accordingly, polypropylene yarns provide greater coverage per unit weight and, in turn, per unit cost. Also contributing to the growth in acceptance of polypropylene yarns are various yarn configurations and features of carpet construction developed over the years that compensate to some extent for the yarns' lower resilience.
In the area of carpet manufacture, elements of construction that compensate somewhat for poor resilience include so-called loop pile constructions, low pile heights and high tuft densities. In loop pile constructions, face yarn tufts that form the carpet's pile surface are left uncut, such that the tufts are disposed in loops on the pile surface. Other things being equal, looped tufts have greater resistance to compression and better recovery than cut pile tufts. Low pile height limits the effect of compressive forces, in any event, by providing shorter tufts to compress. High tuft density, that is, many tufts per unit area of pile surface, makes for close spacing of tufts to one another such that the tufts and their fibers provide support to neighboring tufts and fibers to thereby resist and recover from compression.
In terms of yarn configuration, twisted yarns normally are more resilient than untwisted yarns, with tighter twist and greater twist retention providing greater resilience, other things being equal. Levinstein, The Complete Carpet Manual, 1992, pp. 44-45. It is also known that twist retention can be improved by subjecting yarns to bulking treatments, such as texturizing with fluid jets or crimping, before or after twisting. Those treatments are normally conducted primarily to impart bulkiness and texture to yarns by creating whirls, loops, entanglements, waviness, kinks and crimp in their filaments. Through such interaction of the textured filaments, twist retention in twisted yarns, as well as resilience of even untwisted yarns, are typically improved. Heatsetting often is employed to set, or lock in, twist and bulk. As an example of these types of yarn configurations, U.S. Pat. No. 4,290,378 (1981) of Monsanto discloses "bulky, loopy, heatset, tangled, twisted singles yarn" which may be composed of polyamides (nylons), polyolefins, polyesters and polyacrylonitriles. The yarns are said to have exceptional column strength, resistance to bending and untwisting and to be useful in cut and loop pile carpets. So-called blended yarns, made up of filaments of greater and lesser resilience, e.g., nylon and polypropylene, also have been proposed for the purpose of obtaining yarns with greater resilience than that of yarns composed entirely of the less resilient filaments, as noted in U.S. Pat. No. 3,295,308 (1967) of Eastman Kodak. Yarns composed of so-called bicomponent fibers, such as sheath-core fibers having a core of nylon surrounded by a sheath of polypropylene, also have been proposed as a means to combine the resilience of nylon with the superior properties of polypropylene in other respects.
As a result of some of these yarn configurations and carpet constructions, together with polypropylene yarns' price and performance advantages, polypropylene yarns have gained in acceptance as face yarn for commercial carpets. Of course these techniques do not improve resilience of polypropylene fibers per se, nor do they significantly close the gap between nylon and polypropylene in terms of fiber resilience or pile height recovery of tufted carpets. Rather, both types of yarns benefit from use of carpet construction, yarn configuration within a carpet and fiber-to-fiber interaction within yarn tufts to improve crush resistance. Despite growth in acceptance of polypropylene yarns for commercial carpet, nylon yarns remain dominant.
In residential carpets, the cost and performance advantages of polypropylene yarns over nylon yarns also would be beneficial. As in the case of commercial carpets, superiority of polypropylene yarns over nylon yarns for residential carpets, except in terms of crush resistance, has been widely recognized. Chicago Tribune, Nov. 21, 1993, section 15, page 6. However, in this application, unlike commercial carpets, carpet construction is less useful for masking lower resilience. Loop pile surfaces and low pile heights, while effective and aesthetically satisfactory in commercial carpets, do not provide the plush, luxurious look and feel preferred by homeowners and are seldom used in residential carpets. High tuft density does contribute to a luxurious appearance. However, taken together with the normally greater pile heights of residential carpets, tuft densities high enough to improve compressional recovery due to the supporting effect of adjacent yarns and fibers are so high that they tend to limit utility of polypropylene yarns as face yarn to relatively heavy carpets, e.g., 40 or more ounces per square yard, at the expensive, upper end of the range of commercially available styles and weights.
From 1986 to 1992, the share of polypropylene yarns used as residential carpet face yarn did increase to about five %. Impetus for the increase was provided by Amoco Fabrics Company's Genesis.TM. Carpet certification program. As reported in "Polypropylene Strikes Back," Forbes, Aug. 7, 1989, page 122, resilience of Genesis.TM. Carpets was attributed to twisting continuous filament yarns composed of polypropylene fibers and setting the twisted yarns with heat, together with establishing minimum weight and density specifications and optimized tufting patterns for carpets. The program was limited to the premium end of residential carpets, in part to maximize the resilience benefits of high tuft density, but also in an attempt to dispel polypropylene yarns' image as unsuitable for carpet face yarn or suited only for "low-end" carpets. Despite being so-targeted, and despite receiving considerable advertising and promotional support, the program was discontinued in 1992 because demand for the yarns did not meet expectations. At present, polypropylene yarns' use as face yarn for residential carpet is minimal.
Thus, despite both lower cost and superior performance in a number of respects, polypropylene yarns' growth as face yarn for carpets has been and continues to be hampered by its low resilience. In residential carpets, polypropylene yarns account for only a meager portion of total face yarn. Even in commercial carpets, the acceptance achieved by polypropylene yarns has been the subject of skepticism. Investext.TM., U.S. Pat. No.1,127,564, p. 6 (Jun. 1, 1991).
Beyond particular yarn configurations and modified carpet constructions, attempts to improve polypropylene fiber resilience per se have been reported over the years. It must be recognized, however, that carpet thickness retention and pile height recovery from compressive forces involve complicated interplays among carpet construction, fiber-to-fiber interactions within and among yarn tufts, and fiber and yarn structures, properties and configuration. Furthermore, the bending forces to which carpet fibers and yarns are subjected during normal use normally involve nonuniform compression and stretching of yarn tufts and their fibers; results of yarn testing typically correlate only loosely, if at all, with actual carpet performance. In addition, many properties of fibers and yarns, including not only physical properties but also aesthetics such as appearance and hand, tend to develop over the entire course of their manufacture, including spinning, drawing, and, if conducted, bulking and heatsetting or annealing. Consequently, attempts to improve properties by changing a given process step may require compromises in one or more other steps and/or properties to achieve a balance of overall yarn properties and process efficiency. In view of these factors, it will be appreciated that improvements in fiber properties or manufacture often are difficult to translate into improved carpet performance and that the broad range of interrelationships among fiber and yarn manufacture, their configurations and properties, and carpet performance makes attainment of improved carpet performance through fiber and yarn modifications imprecise and unpredictable. This is aptly demonstrated by prior art related to heat treating or annealing treatments aimed at polypropylene fibers and yarns.
In an early patent to DuPont, U.S. Pat. No. 3,152,380 (1964), the problem of deficient resilience of polypropylene fibers and its negative impact on their use as carpet yarns and stuffing was recognized and a two step process of drawing and heatsetting fibers was proposed as a solution. The patent proposes drawing "as-spun" fibers, defined as fibers as they first solidify on emergence from a spinneret, at a draw ratio of at least 1.5:1, preferably 3:1 to 10:1, and a temperature of at least 80.degree. C., and then heating the drawn fibers in an untensioned state at a temperature of at least 140.degree. C. but below the melting point of the fibers for at least one second. Compressional recovery of the fibers, calculated from recovered height of a yarn plug 24 hours after a one minute exposure to a 10,000 psi load relative to height of the initial plug compressed by a sixteen gram wooden dowel, is described as at least 65%, with values of 65-100% (as opposed to 15-20% for untreated fibers) reported in the patent's examples. The patent also notes that recoveries generally increase with increasing heat treatment temperatures. However, in contrast to improvements reported in the patent's yarn testing examples, carpet testing results show far less improvement (15-20%) as well as accelerated loss of pile height retention at higher levels of foot traffic for treated carpet as compared to untreated carpet. The patent's treated yarns with high compressional recoveries also show excessive shrinkages (30-54%) and, as seen from a comparative example using nylon yarn, even the best of the treated polypropylene yarns had a compressional recovery (100%) of only about two-thirds that of untreated nylon yarn (154%) by the patent's test.
D. R. Buchanan, "Elastic Deformation And Fiber Structure In Polypropylene," date and source unknown, presents comparisons of as-spun, hot-drawn and annealed polypropylene fibers in terms of molecular orientation, crystal structure and tensile recovery. The author's conclusion is that extremely high levels of elastic recovery from large tensile strains were exhibited by fibers annealed for 30 minutes at 127 to 154.degree. C., either under sufficient tension to maintain constant length or relaxed, and that this could be attributed to significantly improved regularity of the supermolecular structure of the fibers, including highly regular crystallite shape and minimum crystal length of 140-150 .ANG.. The annealed fibers reported by Buchanan exhibited tensile recoveries from 30% extension of 91-96%, as compared to 68-86% for hot-drawn fibers and 56-72% for as-spun fibers. Similarly, U.S. Pat. No. 3,256,258 (1966) of DuPont attempts to correlate crystalline structure of polypropylene fibers with improvements in recovery from tensile forces. More specifically, the patent discloses tensile recoveries at 25% elongation of at least 82% in respect of polypropylene fibers characterized by gamma orientation, as indicated by a gamma intensity ratio greater than 0.6, and a heat stable orientation angle of 10.degree. to 30.degree.. Fibers having such features are said to be prepared by melt spinning filaments under conditions vaguely defined as those that afford actual or potential gamma orientation, orienting the filaments to an extent that provides an orientation angle of 10.degree. to 55.degree. and heat treating the filaments at 105-160.degree. C., with 130-140.degree. C. reported to give best results.
While the Buchanan paper and the DuPont '258 patent report improved recoveries from large tensile strains as a result of annealing, neither purports to investigate effects of those improvements on carpet performance. In any event, improved tensile recovery does not lead to or suggest improved resilience because tensile recovery testing measures recovery from stretching or extension while resilience of fibers and yarns depends on bending and compressional recovery. In this regard, the long-recognized superiority of nylon carpet face yarns over polypropylene carpet face yarns in terms of resilience stands in sharp contrast to published works showing that polypropylene yarns are better than nylon yarns in comparative tensile recovery testing. J. C. Guthrie, "The Bending Recovery Of Various Single Fibres," Textile Institute Paper presented to the Textile Institute Physics Group Conference, April, 1970, pp. 615-627. Guthrie reports poor correlation between tensile and bending recoveries for both nylon and polypropylene yarns as does B. M. Chapman, "Bending Stress Relaxation and Recovery of Wool, Nylon 66, and Terylene Fibers," J. Appl. Sci., Vol. 17, pp. 1673-1713, 1975. Guthrie also reports that the relatively low bending recoveries of "as-received" polypropylene fibers reflect their poor carpet performance and stand in contrast to the "much higher" elastic-recovery values obtained by measuring tensile recovery. It follows that the increased tensile recoveries of the heat treated polypropylene yarns reported in Buchanan and the DuPont '258 patent are not predictors of improved resilience of fibers and yarns or of better compressional recovery of carpets.
Guthrie also reports bending recoveries for polypropylene fibers in "as-received" condition; in so-called "straightened" condition with crimp removed by five minutes of heating at 120.degree. C. under tension sufficient to approximately straighten the fibers; and straightened and then relaxed by immersion in water at 95.degree. C. for fifteen minutes. Recoveries one minute and one day after bending are reported as 60.1% and 74.4%, respectively, for straightened fibers, 52.1% and 59.2%, respectively, for straightened and relaxed fibers and 32.0% and 39.1%, respectively, for as-received fibers. Recoveries after repeated bendings also are reported, with straightened fibers routinely showing better recovery than the crimped, as-received ones by 19-35%.
In contrast to Guthrie's teaching that heat treating polypropylene yarns under tension to remove crimp improves bending recovery over that of the untreated, crimped fibers, U.S. Pat. No. 3,686,848 (1972) and its counterpart British Patent Specification 1,384,121 (1975), both to Uniroyal, Inc., are directed to deliberately imparting, and heat treating to permanently set, a particular crimp to obtain polypropylene yarns of improved resilience in terms of ability to recover original dimensions after release of compressive stress. The yarns are said to be characterized by an essential combination of properties that includes textured tenacities of less than 2.5 grams per denier, so-called "crimp permanence" of 20-70, filaments having 6-20 crimps, other than helical or sharp edge angular crimps, per inch, with at least 75% of the crimps being arcuated, three-dimensional crimps, and at least 80% of the filaments exhibiting substantially no plastic deformation. According to these Uniroyal references, the improved polypropylene yarns are prepared by melt spinning polypropylene resin into filaments, bringing the filaments together to form yarn, drawing the yarn at a draw ratio, defined as ratio of drawn length to undrawn length, below 2.5:1 according to the U.S. patent or 3:1 according to the British specification, crimping the yarn as described above and permanently setting the crimp by heat treating the yarn with a minimum of tension in a highly compacted state at 121.degree. C. to just below the softening point of the filaments (desirably at 129-138.degree. C.; 146.degree. C. is the highest temperature reported) for a time sufficient to permanently set the crimp (15 minutes is the only time period quantified). Examples 3 and 4 of each patent present results of simulated and actual traffic testing of level loop carpets tufted with such yarns. In example 3, carpets tufted with such yarns and with nylon yarns, each of 4000 denier, and commercially available level loop carpet tufted with standard 4000 denier polypropylene yarns were tested by measuring pile heights after repeated compressions by application of seven psi over an area 11/4 inch in diameter at a rate of 1380 cycles per hour, with percent matting calculated as a percent of initial pile height represented by pile height at unspecified times after various numbers of cycles. The nylon- and heatset, high crimp permanence polypropylene-tufted carpets were comparable, with both somewhat better than the commercial polypropylene carpet, after 1000 and 3000 cycles. Nylon was slightly better than the heatset, high crimp permanence polypropylene, and both were somewhat better than commercial polypropylene, after 10,000 and 20,000 cycles. Actual carpet testing reported in example 4 is described as showing criticality of the combination of tenacity, draw ratio, crimp permanence and heatsetting in achieving improved resilience; however, a comparative carpet sample tufted with polypropylene yarns prepared with low crimp permanence and without heatsetting is reported to have performed almost as well as a heatset, high crimp permanence sample. As with the DuPont '380 patent, the improved yarn performance reported in these Uniroyal patents far exceeds any demonstrated improvement in carpet performance.
Polypropylene fibers of improved resilience in terms of height of recovery of yarn plugs from compression are reported in U.S. Pat. No. 3,680,334 (1972) to Erickson and Buchanan, in Canadian Patent 957,837 (1974) to Newton and Buchanan (in both cases the same Buchanan who authored the paper discussed previously) and in European Patent Application 0 330 212 (1989) of Wishman et al., all originally assigned to Phillips Petroleum Company. In the two patents, resilience improvements are attributed to reordering of fiber crystal structure so that it is characterized by relatively intense diffraction in the fiber axis direction with maximum diffracted intensity at an angle of 20 minutes such that calculated value of long periods is at least 160 .ANG. and calculated crystal length is 125-200 .ANG., preferably 140-160 .ANG., as determined by small angle X-ray diffraction. Very little additional resilience improvement is said to result from crystal lengths of 170 .ANG. or higher. Reordering of fiber crystal structure is said to be achieved by treating fibers with saturated steam for 0.01-2 seconds under tension at 10-35.degree. C. below the melting temperature of the polymer constituting the fibers, and, in the case of polypropylene homopolymer, preferably at 135-160.degree. C. Short treatment times are said to be necessary because they favor local melting and reorganization of zones of crystal imperfection and because longer treatment times promote soiling of the fibers and yarns and carpets made therefrom. Heating of polypropylene fibers in an air oven for 30 minutes to an hour at 150.degree. C. is also said to give the same crystal size and general structural characteristics but with greatly deteriorated resistance to soiling. In addition to compressional recovery of yarn plugs, the Canadian patent reports thickness retention of loop pile carpets tufted with its treated yarns and comparative, untreated yarns. In a dramatic illustration of the difficulty in translating yarn properties to carpet performance, Table I of the patent shows treated yarns with two-to-three times greater plug height recovery than the untreated yarn (0.636 inch vs. 0.244 inch) but carpet testing results in Table II show so little improvement in performance (80% vs. 76%) as to be of virtually negligible effect.
Still in search of fibers and yarns with improved resilience some fifteen to twenty years after the Erickson, Newton and Buchanan patents discussed above, Phillips' 1989 Wishman et al. application proposes yet another solution. Wishman et al. is directed to resilient polypropylene fibers, said to be suitable for carpets and upholstery, prepared by spinning and drawing polypropylene fibers under conditions that produce sufficient crystallinity to withstand subsequent heat treatment, including a draw ratio of at least 3:1, imparting to the fibers a sharp edge angular or so-called two-dimensional type crimp, and heat treating the fibers to permanently set the crimp. (It will be noted that Wishman et al.'s minimum draw ratio of 3:1 exceeds or equals the maximum draw ratio according to the previously discussed Uniroyal references; they also set mutually exclusive requirements for crimp configurations.) Heat treating is conducted at about 280.degree. F. (138.degree. C.) to just below the softening point of the fibers, reported as 320-329.degree. F. (160-165.degree. C.), and preferably at about 284.degree. F. to about 315.degree. F. (140-157.degree. C.), for a time ranging from five seconds to eight minutes depending on heat transfer capability of the heat treating system and openness of the fiber bundle. Unlike the earlier patents to Phillips, Wishman et al. does not attribute improvements to changes in fiber crystal structure but, rather, to permanently setting a particular crimp in the fibers. Staple fiber prepared according to Wishman et al. has achieved some measure of success, particularly in certain automotive applications, such as door panel fabrics, and for certain apparel fabrics. Indeed, in the apparel field, spun yarns composed of such staple fibers have been woven or knit, alone or with other spun and/or filament yarns, e.g. polyester, cotton, wool, nylon, other polypropylene yarns, to yield various woven fabrics, e.g., denims, hopsacks, twills, and knits, e.g. circular, warp, flatbed and sliver knits and knitted fleece, having beneficial and interesting characteristics. For example, woven and knit fabrics have been prepared from combinations of such spun yarns and cotton yarns to yield garment fabrics with improved comfort due to a so-called "push-pull" effect resulting from polypropylene's hydrophobicity and cotton's water absorbency and improved appearance retention resulting from bulk retention and resiliency of the polypropylene yarns. However, such yarns are not used in commercial or residential carpet nor have continuous filament yarns according to the reference met with commercial acceptance. Indeed, in the area of continuous filament yarn, Phillips discontinued its business, including carpet face yarn, by 1992 after a two year phaseout. Spartanburg Herald Journal, Nov. 2, 1991.
Improved resilience also was an aspect of the now-discontinued, Amoco Fabrics Company Genesis.TM. Carpet certification program that took place from 1988 until 1992. Genesis.TM. Carpet face yarns were made by melt spinning polypropylene homopolymer resin from delta-shaped spinning orifices, gathering the filaments into yarns, drawing the yarns at draw ratios of about 3.5:1, air jet-texturing the yarns to impart bulk and texture, twisting the yarns 4.5 twists per inch, steaming the yarns at 98.degree. C. for 12 seconds and then locking in the twist by heatsetting at about 132.degree. C. for 32 seconds. Resilience of Genesis.TM. Carpet face yarns tested according to the Plug Crush Recovery Test, a compressional recovery test described in detail hereinbelow, is about 75%. While this is on the high side of Plug Crush Recoveries of conventional polypropylene carpet face yarns, it still falls well short of nylon (Plug Crush Recovery=85-90%). After discontinuing the Genesis.TM. program, Amoco exited the carpet face yarn business. Atlanta Constitution, Sep. 26, 1992, Business Section, p. C3.
From the foregoing, it is evident that a variety of improved yarns and yarn manufacturing processes has been proposed or utilized, with a broad range of results reported. Reported improvements in resilience of yarns, when achieved, have been accompanied by severe losses of other important properties, including excessive shrinkage, development of fused or "crispy" surfaces, and texture, loss of hand and other aesthetic properties. In any event, such improvements as have been reported show little or no positive effect on carpet performance. Moreover, while heatsetting is a common feature of many prior proposals, it also can be seen that the combination of other process conditions and steps, e.g., heatsetting with or without tension, draw ratio, type and level of crimp, if any, and others, has a major influence on attainment of improved results. Noteworthy in this regard are Phillips' Wishman et al. European application and the Uniroyal references which report similar results yet set contradictory requirements for draw ratio and type of crimp. Adding to the confusion, the U.S. patent to Uniroyal and its British counterpart, while stressing criticality of various yarn properties and process features, do not even agree on maximum draw ratio. Further, in contrast to Wishman et al.'s and the Uniroyal references' attempts to utilize crimp to improve resilience, Guthrie reports better bending recovery for straightened fibers than for crimped fibers. Some of the proposals discussed above also purport to establish relationships between crystalline structure and physical properties; however, those that do so in the context of improved recovery from tensile deformations are unreliable as a guide for improving resilience because of the well-established lack of correlation between such properties. In any event, no two of the references even measure the same crystallinity parameters. Perhaps most significantly, despite availability of these proposals and their teachings and results, many for twenty-to-thirty years, polypropylene yarns' use as carpet face yarns remains limited due to resilience and two formerly significant propylene polymer carpet yarn producers, Amoco and Phillips, have left that business.
Other proposals for polypropylene fibers and yarns of improved resilience have been advanced over the years. U.S. Pat. No. 3,286,322 (1966), also to Phillips Petroleum Company, proposes immersing BCF polypropylene yarns in a bath of crosslinkable polyfunctional monomers and then exposing the monomer-saturated yarn to high energy radiation to crosslink the monomer and polypropylene and thereby improve resilience. Canadian Patent 787,824 (1968) of Union Carbide Corporation proposes polypropylene fibers having cross-sections with three sharp angles of less than 90.degree. to distribute mass over greater area and away from the fiber axis, thereby increasing bending moment and, in turn, resilience of the fibers above that of round cross-sectioned filaments. Neither proposal was ever implemented in any commercial operation so far as is known.
Thus, despite the longstanding search for improved resilience, the variety of approaches pursued by the polymers, fiber and yarn, and carpet industries over the years, and propylene polymer yarns' superiority over other carpet yarns in virtually every respect other than resilience, the fact remains that propylene polymer yarns remain a distant second to nylon as carpet face yarns, especially in residential carpets. Judged by actual experience in the carpet industry, the proposals discussed above must be regarded as unsuccessful and the longstanding search and long felt need for propylene polymer fibers and yarns of improved resilience continue.